1. Field of the Invention
The present invention relates to a mixing head adapted to be connected with and to form part of an extruder plastication screw. More particularly, the present invention relates to a mixing head that includes a plurality of external spiral channels defined by a plurality of spaced helical flights, the flights each having peripherally positioned undercuts defining flow passageways between the respective spiral channels to enable improved mixing of the plasticated material to occur in order to provide improved homogeneity of the extruded material that leaves the extruder.
2. Description of the Related Art
Extruder plastication screws typically contain one or more radially outwardly extending helical flights that define one or more helical flow channels between the respective flights. As polymeric material in either powdered or pelletized form is introduced into a material inlet provided in the extruder barrel within which the screw is rotatably carried, the solid material is gradually carried along the screw from the material inlet to the material outlet. While the material is being conveyed along the screw it is heated in order to soften it to permit it to be extruded through a suitable extrusion die positioned at or adjacent to the outlet of the extruder barrel. Heating of the material is accomplished by external heat sources positioned on the outer surface of the barrel, such as by means of a series of annular electrical resistance heating elements, and also by internal heating of the material by virtue of mechanical working of the material as it progressively moves along the extruder screw.
Frequently, additives such as colorants, anti-static compounds, and the like, are added to the polymeric material to provide desired properties and appearance. Such additives must be intimately intermixed with the polymeric material before the material exits from the extruder.
Over the years, many different types of extruder screw flight configurations and screw constructions have been proposed for improved mixing of the material to be extruded. For example, in U.S. Pat. No. 3,427,003, which issued on Feb. 11, 1969, to R. Schneider et al., there is disclosed a mixing head having a generally cylindrical body that carries a set of inner helical flights of one hand, and a set of outer flights surrounding the inner flights but of the opposite hand. The Schneider et al. construction provides a plurality of helices, defined by the respective inner and outer flights, the helices communicating with each other and serving to provide greater mixing of the material being extruded as the material is conveyed along the mixing head.
In U.S. Pat. No. 3,486,194, which issued Dec. 30, 1969, to C. E. Parks, there is shown and described a plastics extruder screw that incorporates a mixing head in the form of an elongated torpedo at its forward or discharge end. The torpedo includes a plurality of longitudinally staggered, circumferentially spaced, radially extending flight segments that can either be positioned so that they are aligned in a longitudinal direction or alternate rows of flight segments can be inclined relative to each other.
In U.S. Pat. No. 4,779,989, which issued Oct. 25, 1988, to Robert A. Barr, there is disclosed a plastics extruder screw having a mixing head or element at its downstream end. The mixing head includes a plurality of interrupted helical grooves, and the adjacent portion of the surrounding barrel includes a plurality of recessed helical grooves of opposite hand to the grooves in the mixing head. The disclosed structure provides a plurality of individual recessed cavities, some of which are formed in the surface of the barrel and others of which are formed in the surface of the mixing head, to permit transfer of the plasticated material between the respective cavities as the material is being conveyed, to thereby improve the homogeneity of the extruded material.
In U.S. Pat. No. 3,941,535, which issued Mar. 2, 1976, to Louis F. Street, there is shown an extruder plastication screw having an intermediate mixing section that includes a plurality of helical flights, each of the flights having a plurality of peripheral slots spaced from each other along their respective flights. The slots each extend across the respective flights in a direction that is substantially perpendicular to the longitudinal axis of the screw, and the slots progressively increase in depth in the direction toward the discharge end of the screw. Again, the passageways defined by the slots are intended to permit flew of plasticated material to take place between adjacent channels, and thereby promote mixing of the material being extruded.
Although the prior art devices serve to promote some degree of improved mixing, as compared with a conventional screw not having such mixing-promoting devices, a number of the structures proposed in the prior art are difficult to manufacture, and therefore are of high cost. Additionally, some of the proposed approaches impose substantial shearing forces on the plasticated material, which can cause excessive and undesired localized heating of the material, possibly causing degradation of the material.
It is an object of the present invention to overcome the deficiencies of the prior art structures, and to provide an improved extruder mixing head that is effective to promote mixing of the plasticated material for improved homogeneity, and at the same time one that is of relatively simple structure, and is therefore economical to produce.